3 Making Pancakes, ContinuedAssume that you have enough flour and baking powder. You can use the number of eggs you have to determine the number of pancakes you can make.Tro's “Introductory Chemistry”, Chapter 8

7 Tro's “Introductory Chemistry”, Chapter 8PracticeAccording to the following equation, how many moles of water are made in the combustion of 0.10 moles of glucose (C6H12O6)?C6H12O6 + 6 O2 ® 6 CO2 + 6 H2OTro's “Introductory Chemistry”, Chapter 8

8 Making Molecules Mass-to-Mass ConversionsWe know there is a relationship between the mass and number of moles of a chemical.1 mole = Molar Mass in grams.

14 More Making Pancakes, ContinuedEach ingredient could potentially make a different number of pancakes.But all the ingredients have to work together!We only have enough flour to make 15 pancakes, so once we make 15 pancakes, the flour runs out no matter how much of the other ingredients we have.

15 More Making Pancakes, ContinuedThe flour limits the amount of pancakes we can make. In chemical reactions we call this the limiting reactant.Also known as limiting reagent.The maximum number of pancakes we can make depends on this ingredient. In chemical reactions, we call this the theoretical yield.Tro's “Introductory Chemistry”, Chapter 8

18 More Making PancakesAssume that in making the pancake, you spill some of the pancake mix, burn a pancake, drop one on the floor, or other uncontrollable events happen so that we only make 11 pancakes. The actual amount of product made in a chemical reaction is called the actual yield.We can determine the efficiency of making pancakes and this is known as the percent yield.

19 Theoretical and Actual YieldAs we did with the pancakes, in order to determine the theoretical yield, we should use reaction stoichiometry to determine the amount of product each of our reactants could make.The theoretical yield will always be the least possible amount of product.The theoretical yield will always come from the limiting reactant.The actual yield of product will always be less than the theoretical yield.Tro's “Introductory Chemistry”, Chapter 8

20 Measuring Amounts in the LabIn the lab, our balances do not measure amounts in moles, unfortunately, they measure amounts in grams.This means we must add two steps to each of our calculations: first convert the amount of each reactant to moles, then convert the amount of product into grams.Tro's “Introductory Chemistry”, Chapter 8

22 Since the percentage yield is < 100, the answer makes sense.Example 8.6—When 11.5 g of C Are Allowed to React with g of Cu2O in the Reaction Below, 87.4 g of Cu Are Obtained. Cu2O(s) + C(s)  2 Cu(s) + CO(g), ContinuedSolution:The smallest amount is g Cu, therefore that is the theoretical yield.The reactant that produces g Cu is the Cu2O,Therefore, Cu2O is the limiting reactant.Check:Since the percentage yield is < 100, the answer makes sense.

25 Tro's “Introductory Chemistry”, Chapter 8Enthalpy ChangeWe previously described processes as exothermic if they released heat, or endothermic if they absorbed heat.The enthalpy of reaction is the amount of thermal energy that flows through a process.DHrxnTro's “Introductory Chemistry”, Chapter 8

26 Sign of Enthalpy ChangeFor exothermic reactions, the sign of the enthalpy change is negative when:DH = ─Thermal energy is produced by the reaction.The surroundings get hotter.For the reaction CH4(s) + 2 O2(g)  CO2(g) + 2 H2O(l), the DHrxn = −802.3 kJ per mol of CH4.For endothermic reactions, the sign of the enthalpy change is positive when:DH = +Thermal energy is absorbed by the reaction.The surroundings get colder.For the reaction N2(s) + O2(g)  2 NO(g), the DHrxn = kJ per mol of N2.